Brakes used in hoisting devices are critical to the operation of the hoisting device both in terms of economy and safety. The malfunction of a brake may lead to dropping the load, whereby other parts of the hoisting device may be damaged and there is a risk that the persons working close by will get hurt.
Brakes used in hoisting devices are conventional disc brakes, in which disc-like friction material rotating along with the motor axle presses between an anchor plate and a friction plate and deceleration takes place.
The friction surface wears during deceleration. If the friction surface wears out, the braking effect of the brake decreases significantly and the brake can no longer operate as designed. Thus, the brake malfunctions.
The condition of a brake is typically monitored manually by measuring the thickness of the friction surface with a slide gauge. So as to be able to measure the friction surface, a possible brake casing thus needs to be dismantled, and after the measurement and the possibly required brake maintenance, the casing needs to be closed again. Brake monitoring thus entails manual labour that is time-consuming and subject to errors. In addition, in hoisting devices, in which the brake is high up, as in harbour cranes, there is a risk that the maintenance person may fall, and taking this into account may further add to the time required to perform maintenance.
During the checking of the brake of the hoisting device, the production machine to which the brake is mounted is not available for use for its conventional work. A harbour crane under brake maintenance, for instance, is then out of use during the checking of the brake. However, in terms of economy, the downtime of an expensive apparatus like a harbour crane should be kept as rare and short as possible. Therefore, it is desirable that breaks in the operation of production machines be as short as possible. By scheduling the production breaks, it is possible to shorten the downtime, as the maintenance work can be planned better.
In terms of safety, the wear of a hoisting device brake causes a risk by increasing the probability of a malfunction of the brake, which may cause economic losses, if the hoisting device or its parts break or people in the vicinity of the hoisting device are hurt, or damage is caused to the production line or other property. For example, if the brake does not engage, the load of the hoisting device may fall. In another example, if the brake does not release, the motor of the hoisting device may run against the brake and the brake may overheat and even explode. Running against the brake refers to running the motor with the brake engaged, when the motor is normally switched off when the brake is engaged. Running against the brake may take place during the normal use of the brake, when the motor continues to run after the brake has been engaged.
Several brakes are typically used in hoisting devices to stop a load being handled and to keep it in place. In a hoisting device, the load can be moved in many different directions of movement. For instance, in harbour cranes, a container may be lifted and lowered using four ropes that connect to the corners of the container. The motor of each rope can be decelerated separately, which means that a number of brakes corresponding to that of the motors are needed, for instance four. In another example, in a bridge crane, the load is moved by means of a carriage, bridge and hoist in the directions of movement they permit. A safe movement of the load with the hoisting devices may, thus, require that the several brakes moving the load are in order. The maintenance of several brakes is challenging due to the number of brakes in the hoisting devices and/or the size of the hoisting devices. The challenge of the maintenance is further increased by the differences in the brakes, which may be due to the differences in the models of the brakes, when they originate from different manufacturers or are different models of the same manufacturer, for instance. Typically, the brakes of one and the same manufacturer may differ from each other in size and/or supply voltage.